Swelling clays can be a major mechanism of formation damage due to loss of mobility of hydrocarbon fluids in the formation. Migrating clays and other fines also cause permeability damage. When clays and other fines are disturbed, they may begin to migrate within the production stream and, too frequently, encounter constrictions in the capillary, where they bridge off the capillary and severely diminish the flow rate of hydrocarbons to the wellbore. The change in the nature of the water present may cause the fines to disperse or come loose from adhesion to capillary walls, usually resulting in the migration of the fines through the formation, where plugging can occur in smaller pore throats. Sometimes the loss of permeability observed is due to clay swelling with the relatively fresh water, with or without migration, but often clay swelling is accompanied by migration of fines. Non-swelling clays may also respond to the foreign water and begin to migrate.